Orthofermions versus Gutzwiller projection operator approach for the infinite U Hubbard model

A comparison of two well-known approaches for strongly correlated electron systems, namely, nested Bethe ansatz implemented through orthofermion algebra and Gutzwiller projection operator formalism, is made by calculating the energy spectrum of 1D infinite U Hubbard model for a finite system consisting of three particles on a four site anisotropic closed chain. It is shown that orthofermion algebra always leads to at least an eight hold degeneracy in the energy spectrum corresponding to all 2³ spin configurations, consistent with the nested Bethe ansatz solution leading to a N²-fold degeneracy of energy levels of an N electron system. Such a degeneracy is absent in the Gutzwiller projection operator approach. This finding shows the limitations of the Gutzwiller projection method and at the same time the relevance of orthofermion approach for the infinite U Hubbard model.     

Electron spin resonance of nanoporous carbon with embedded cobalt clusters

Cobalt clusters were embedded into a nanoporous carbon powder (with pores about 2 nm in size) prepared from a B₄C carbide powder. Electron spin resonance (ESR) measurements were carried out within a broad temperature range. At all temperatures, the ESR spectrum consisted of two overlapping resonance Lorentzian lines. The temperature dependences of the integrated intensities and linewidths and of the resonance fields were determined. A theoretical analysis of these dependences shows that they can be described in terms of the theory of a disordered magnetic medium with two spin systems with different properties.     

Optical characteristics of MBE grown GaMnAs embedded with MnAs clusters

Photoluminescence (PL) measurements of the GaMnAs layers embedded with MnAs clusters have been performed. It was shown that the presence of MnAs clusters in the semiconducting matrix leads to appearance in the PL spectra a broad peak with local maximums at 1.36 and 1.33 eV, which are related with the defects generated in the phase separation process. The effect of the MnAs clusters on the temperature dependent band gap of GaMnAs was also observed.     

Grayscale projection of finite size fractal clusters

The European Physical Journal E - Microscopy techniques are suitable to obtain structural information of colloidal clusters with high resolution, but yield only a two dimensional projection of the...     

Grayscale projection of finite size fractal clusters

Microscopy techniques are suitable to obtain structural information of colloidal clusters with high resolution, but yield only a two dimensional projection of the objects. When imaging finite size objects with fractal properties, such as clusters of colloidal particles, this projection process has to be taken into account for the calculation of the fractal dimension. In this paper we present a technique to calculate the fractal dimension of finite size clusters with fractal properties using grayscale projections such as images obtained by X-ray microscopy. The grayscales are interpreted as different occupation counts within a projection. It is shown, that the radial distribution of these occupation counts varies with the fractal dimension d of the cluster. Using the radius of maximum occupation probability the fractal dimension up to 2.2 of finite size clusters can be calculated. The theoretical predictions are verified by test calculations employing numerically generated clusters.     

Nonlinear optical spectroscopy of embedded semiconductor clusters

Received: 20 September 1998 / Revised version: 11 January 1999     

Linear and non-linear response of embedded Na clusters

We investigate Na clusters embedded in Ar matrices. The surrounding Ar atoms are modeled in terms of their dynamical polarizability and the strong electron repulsion. The calibration of the model is discussed. First results for the non-linear optical response of the Na clusters are presented for the test case of Na₈ embedded in Ar ensembles of different sizes. It is shown that blue shift through core repulsion and red shift through dipole polarizability counterweight each other to the end that very little global shift is seen in the spectra. This feature persists to all excitation strengths considered. There are, however, detailed effects, such as for example the Landau fragmentation of the Mie plasmon peak. PACS 36.40.Gk; 36.40.Vz; 31.15.Ew     

Implementation of LDA + Gutzwiller with Newton's method

In order to calculate the electronic structure of correlated materials, we propose implementation of the LDA+Gutzwiller method with Newton's method. The self-consistence process, efficiency and convergence of calculation are improved dramatically by using Newton's method with golden section search and other improvement approaches.We compare the calculated results by applying the previous linear mix method and Newton's method. We have applied our code to study the electronic structure of several typical strong correlated materials, including SrVO_3, LaCoO_3, and La_2O_3Fe_2Se_2. Our results fit quite well with the previous studies.     

Coupled plasmon and phonon dynamics in embedded Na clusters

We investigate, from a theoretical perspective, the coupled electronic and ionic/atomic dynamics of Na clusters embedded in Ar matrices. The system is described by time-dependent density-functional theory for cluster electrons and classical motion for Na⁺ ions as well as for Ar atoms. The interaction with the surrounding Ar atoms is modelled by polarization potentials plus core repulsion. We use this model to study coupled electronic and ionic/atomic motion in embedded clusters following a very short laser pulse. For excitations in the non-linear regime, we find clear signs for the coherent coupling of the Mie plasmon resonance with ionic vibrations (phonons). In addition, an incoherent line stretching is observed which can be traced back to the turning point of ionic vibrations. The coupling to the atomic motion of the surroundings leads to a slow and far reaching rearrangement of the matrix. PACS 36.40.Gk; 36.40.Vz; 31.15.EW     

Frequency shift of sources embedded in finite two-dimensional photonic clusters

The frequency (Lamb) shift and local density of states (LDOS) in two-dimensional photonic crystals composed of a cluster of infinitely long circular cylinders is calculated classically using the radiation reaction mechanism. We investigate the frequency shift and LDOS as a function of the size of the cluster and show that, at the edges of the band gap, both quantities can be large and increase in magnitude with cluster size. We explain this in terms of poles of a scattering operator and also show that both the Lamb shift and LDOS are sensitive functions of the shape of the cluster.     

Frequency shift of sources embedded in finite two-dimensional photonic clusters

The frequency (Lamb) shift and local density of states (LDOS) in two-dimensional photonic crystals composed of a cluster of infinitely long circular cylinders is calculated classically using the radiation reaction mechanism. We investigate the frequency shift and LDOS as a function of the size of the cluster and show that, at the edges of the band gap, both quantities can be large and increase in magnitude with cluster size. We explain this in terms of poles of a scattering operator and also show that both the Lamb shift and LDOS are sensitive functions of the shape of the cluster.     

Excited-state relaxation of Ag8 clusters embedded in helium droplets

Neutral silver clusters Ag(N) are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for Ag8, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of (6.89+/-0.09) eV for the vertical ionization potential of Ag8. The influence of the helium matrix on photoemission is discussed.     

Multi-Band Gutzwiller Wave Functions for Itinerant Ferromagnetism

Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The approximate evaluation of these variational ground states becomes exact in the limit of large coordination number. The result allows the identification of quasi-particle band structures for correlated electron systems. As a first application, we summarize a study of itinerant ferromagnetism in a two-band model, thereby elucidating the co-operation of the Coulomb repulsion and the Hund's-rule exchange. Then, we present results of calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis of 4s, 4p and 3d valence electrons. Good agreement with the experimental ground-state properties of nickel is obtained. In particular, the quasi-particle energy bands agree much better with the photo-emission and Fermi surface data than the band structure obtained from spin-density functional theory. Finally, we present results for the variational spinwave dispersion for our two-band model.